Researchers in Canada and Austria found that serotonin transporter protein, a brain chemical that reduces levels of the mood-regulating hormone serotonin around brain cells, is more active during the darker months of the year, and suggested this could be the mechanism that explains why some people feel less happy and lower in energy during the winter.

The study was the work of scientists based at the University of Toronto and the Medical University of Vienna and is published in the 9th September online issue of the Archives of General Psychiatry.

The researchers took PET scans of people’s brains to investigate the relationship between regional serotonin transporter (5-HTT) binding and season. They found that 5-HTT binding was higher during the darker months of the year than the lighter months. Since elevated 5-HTT binding is linked to lower levels of serotonin around synapses, the places where brain cells link up with each other, the researchers suggest they may have found the mechanism that explains seasonal changes in mood and behaviour.

Serotonin is a brain chemical that affects mood and other physiological functions such as energy balance, feeding, sleeping and mating, wrote the authors, and it was already known that these and other serotonin-related conditions followed a seasonal pattern. There is a tendency for example, for people who do not live near the equator to feel happier and more energetic on days when the sun is shining and to feel lower energy and mood during the winter when days are shorter.

But until this study, the underlying mechanism to explain this seasonal change in brain chemistry was somewhat of a mystery. So the researchers decided to investigate how seasonal changes affected the behaviour of the 5-HTT serotonin transporter, since it plays an important role in regulating the levels and reach of serotonin signals.

For the study, the researchers measured indices of serotonin transporter density called “regional serotonin transporter binding potential values” at different times of the year and correlated it to duration of daily sunshine.

They recruited 88 healthy volunteer patients and measured the regional serotonin transporter binding potential values in their brains from December 1999 to December 2003 using positron emission tomography (PET scans). The type of PET scan they used was called carbon 11-labeled 3- amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile positron emission tomography.

The results showed that in all brain regions in individuals scanned in the darker months of fall and winter, the serotonin transporter binding potential values were significantly higher compared to those scanned in the lighter months of spring and summer (P = .01 to .001).

They also showed that as average levels of daily sunshine went down, the serotonin transporter density went up for all brain regions.

The authors concluded that:

“Serotonin transporter binding potential values vary throughout the year with the seasons.”

“Since higher serotonin transporter density is associated with lower synaptic serotonin levels, regulation of serotonin transporter density by season is a previously undescribed physiologic mechanism that has the potential to explain seasonal changes in normal and pathologic behaviors,” they added.

The researchers said the findings could be important in understanding why some people have seasonal affective disorder (SAD) and why people experience seasonal mood changes and depressive episodes in the darker months of the year.

Dr Jonathan Johnston, a lecturer in neuroscience at the University of Surrey, told the BBC that while the study did show a link between serotonin transporter chemical and hours of sunshine, it did not actually explain how sunshine duration changed transporter activity.

“Seasonal Variation in Human Brain Serotonin Transporter Binding.”
Nicole Praschak-Rieder; Matthaeus Willeit; Alan A. Wilson; Sylvain Houle; Jeffrey H. Meyer.
Arch Gen Psychiatry. 2008;65(9):1072-1078.
Vol. 65 No. 9, September 2008.

Click here for Abstract.

Sources: Journal Abstract and summary, BBC.

Written by: Catharine Paddock, PhD